1. Field of the Invention
The present invention relates to a discharge cell for use in an ozone generator and, more particularly, to a discharge cell which is suitable for an ozone generator for semiconductor production.
2. Description of the Background Art
Discharge cells for use in ozone generators are generally classified into a plate type and a tube type. Such a discharge cell, whichever type it falls in, has a pair of electrodes spaced with a discharge space therebetween, and a dielectric layer provided on a surface of at least one of the electrodes. Generation of ozone gas is achieved by passing a raw material gas such as oxygen gas through the discharge space.
It is a conventional practice to employ a glass plate as the dielectric layer for the discharge cell. However, the glass plate dielectric layer generally has a great thickness because glass has a small dielectric constant. For this reason, ceramics which have greater dielectric constants than glass are often used for the dielectric layer. Glass plate dielectric layers will supposedly be superseded with dielectric layers such as of ceramic plates and sapphire plates which have greater dielectric constants and definite compositions as will be described later.
The ozone generators are used in a variety of chemical treatment plants, and introduced into semiconductor production plants. In the case of ozone generators to be employed for formation of oxide films, ashing of resists, cleaning of silicon wafers and the like in the semiconductor production, it is necessary to generate high purity ozone gas containing an extremely small amount of contaminants (metal impurities and particles). To this end, high purity oxygen gas is used as the raw material gas for the generation of ozone gas.
Where the dielectric layer of the discharge cell is formed by sintering a dielectric material such as a ceramic material on the surface of the electrode, a portion of the dielectric layer undesirably has an indefinite composition. Therefore, dielectric layers formed in such a manner are being superseded with dielectric layers such as of preformed ceramic plates and sapphire plates which are highly pure and have definite compositions. Further, a pipe formed of a stainless steel such as SUS316L is used as a pipeline for supplying the generated ozone gas to an application site.
Where high purity oxygen gas is used as the raw material gas, however, the concentration of ozone in the generated ozone gas tends to be reduced with time. This is a critical problem. An effective approach to this problem is to add a trace amount of a catalytic gas to high purity oxygen gas (Japanese Unexamined Patent Publications No. 1-282104 (1989), No. 1-298003 (1989) and No. 3-218905 (1991)).
High purity nitrogen gas is often used as the catalytic gas for prevention of the time-related reduction in the ozone concentration, because the nitrogen gas is readily available for the semiconductor production. However, it has recently been found that, where nitrogen gas is used in combination with oxygen gas, ozone gas to be supplied contains metal impurities, which adversely influence the semiconductor production. This is because, where nitrogen gas is added to high purity oxygen gas for the generation of ozone gas, the generated ozone gas contains nitrogen oxides as side products, which may deteriorate or corrode the interior surface of the stainless steel pipe. As a result, the metal impurities from the stainless steel pipe are deposited in a site to which ozone gas is supplied.
To solve this drawback, Japanese Unexamined Patent Publication No. 8-133707 (1996) proposes addition of a relatively large amount (10 to 20 vol %) of carbon dioxide and/or carbon monoxide as a catalytic gas. The use of a catalytic gas other than nitrogen gas reduces the production of the metal impurities in the site supplied with ozone gas. However, this approach requires an additional pipeline for supplying the catalytic gas to the oxygen gas pipeline, thereby complicating the system. In addition, atoms other than oxygen atoms are contained in the generated ozone gas, so that the composition of a treatment gas may vary. For example, this results in formation of Si--CH.sub.3 bonds in an SiO.sub.2 film. Therefore, this approach is still unsatisfactory.